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dab
Antenna-Theory.com Newbie

Joined: 27 Oct 2014
Posts: 3

 Posted: Mon Oct 27, 2014 3:29 pm    Post subject: limiting coverage area of 2.4 GHz data transmitter/receiver Hi How can I limit coverage area of a 2.4 GHz data transmitter/receiver module from a sphere with about 50 meters radios to a cylinder with about 10 meters length and about 1 to 2 meters radios in base ? thanks in advance dab
E Kafeman
Antenna Theory Regular

Joined: 04 Feb 2013
Posts: 25
Location: Sweden

 Posted: Tue Oct 28, 2014 5:42 am    Post subject: RF waves emitting from an omnidirectional antenna coverage behaves a bit as light emitted from an naked electrical lamp bulb. As for the bulb can antenna have an reflector, resulting in a more directive beam. Compare headlight beam emitting from a car. Signal/light-beam becomes gradually weaker with distance, it will not shine with constant power and then suddenly stop after a certain distance. Practical coverage is depending how strong signal that receiver needs and if its antenna is directive. For an assumed transmitter and receiver-system, TX power can be reduced so nothing will be heard at 50 meters distance and at 10 meters are about half of actual information transferred. A better RX-circuit equipped with antenna with high gain can then maybe still pick up all information with almost no error at 50 meters distance. Beam-forming (directive antennas ) can typical be done with a parabolic dish, Yagi-Uda antenna, tin-can antenna, to mention a few common types. Beamwidth for these antennas is given as an angel (degrees), not as a diameter. As for a directed light-beam, directive radio waves becomes gradually weaker outside of its beamwidth, which is a conical shape instead of a cylindrical. This conical shape is infinite long, but becomes gradually weaker with distance as beam is spread over a bigger area with distance.
dab
Antenna-Theory.com Newbie

Joined: 27 Oct 2014
Posts: 3

 Posted: Tue Oct 28, 2014 7:43 pm    Post subject: Dear Kafeman Thanks for your post. Let me describe the problems I am trying to solve: There are two neighborhoods area, one square about 10 x 10 meters, and another rectangular ones about 2 x 10 meters. I want using two data transmitter/receiver modules, cover this two area, separately (please do not write me about addressing techniques & ...). It is possible to install data modules inside a tin-can as cantenna. I will be appreciated with your comments & suggestions regarding module (or antenna) installation strategy. thanks for your time dab _________ |............|..| |............|..| |............|..| |............|..| the above is a graph from two area
E Kafeman
Antenna Theory Regular

Joined: 04 Feb 2013
Posts: 25
Location: Sweden

Posted: Wed Oct 29, 2014 12:28 am    Post subject:

Software addressing techniques are not relevant in terms of coverage or field strength.
Addressing with aid of antenna polarization is very relevant for resulting coverage where environment, such as ground reflections also plays an important role but guess you already know this.

Besides addressing, what is your definition of coverage and do it also depends on your type of transmission. protocol?
Is it 10 meter coverage without people moving around and maybe causing shadowing of radiation beam? Anything else reflective or absorbing within actual environment?
Think in terms of flashlight beam, what shape do its beam need to have to give even light spread everywhere within your 3-D location, with light level powerful enough for an assumed receiver needs, received by an omnidirectional antenna. Is a such flashlight beam-shape realistic? If not, then is it not easier to achieve coverage with RF waves and antennas.

Any beam will spread a lot caused by reflections. Reflections gives interference with main beam and causes weaker and stronger spots. To avoid too weak spots, without reception, is a TX margin assumed, for BT typical indoor calculation is that TX signal be 10 times stronger then it else need for coverage in free space for same distance.

If a certain spread to neighbors is allowed is distributed antennas a possibility. Let each antenna cover 0.5x0.5x0.5 meter with minimum required signal level, The antenna is placed in center of this volume and radiates omnidirectional. A problem is that radiated polarization must be same for both TX and RX antennas if reliable transmission with low signal levels should be possible.
RF spread to neighbors will be well defined with this antenna system

A similar system is used in subways and car-tunnels, where FM and GSM is retransmitted with aid of leaking coaxial-cables as antennas. As tunnel-walls in most cases are rather reflective do it result in a field of similar strength created along whole tunnel. This type of antenna behaves like a long light-rope but as each part of the rope only need to cover a small part of actual tunnel volume is local field relative weak.

A directive conical beam, many small antennas or leaking coaxial cable do all result in different coverage and resulting in different shape of field that will be spread outside of your wanted coverage volume.

One way to avoid leakage outside of an volume is to total cover this volume with a metallic screen. As it is 2.4 GHz, can it be complicated to block leakage as even minor holes can drastically puncture result. Compare with a microwave oven.

Directive antennas cause a directed beam with gradually weaker signal outside of main beam. There is no practical way except for walls, either reflecting or absorbing to avoid a spread outside of main beam. Any reflecting inside your zone will send signals in other directions, also outside of expected zone.
Are walls an alternative? From what point is actual coverage needs defined or expected? Is it an field strength in free space or should it result in a fully working transmission in a complex environment? Can you create a such shaped field with aid of visible light, that result in a minimum required light level everywhere inside of a volume and still low enough light spread outside of this volume?

Volume can not be a 2D shape and it is probably not free space inside this volume? Is it any existing walls and roof covering that represent volume limits or can it be added?
Can you give an idea what the circumstances actually are and can you imagine how to achieve similar result with aid of a visible light sources, and provide that information, is that a good start. Remember that it is a volume that must be filled, not a 2D shape, also for a light-source.
dab
Antenna-Theory.com Newbie

Joined: 27 Oct 2014
Posts: 3

 Posted: Wed Oct 29, 2014 4:17 pm    Post subject: Dear Kafeman Thanks for your post and time. The leaking cable may work in the situation. Is it a good idea to cut pcb antenna of module and solder it to head of leaking cable? How much resistor (or other circuit) is required to terminate tail of antenna to the shield? (output impedance of the module has designed for 50 ohm loading) What circuit is necessary for protection of module from sharp peaks induced in this long transducer? (In ordinary circuits we only use RC and protection diodes!) Is only grounding the shield of cable antenna to the ground of module is enough? Best Regards dab |-------- |--------| ---------| | the above graph is few centimeter pcb antenna of the module
E Kafeman
Antenna Theory Regular

Joined: 04 Feb 2013
Posts: 25
Location: Sweden

 Posted: Wed Oct 29, 2014 5:35 pm    Post subject: Yes, cut away any other antennas as result else will be hard to predict. An end terminator with same characteristics as the leaky coaxial cable can be used but is often omitted. An end cap is recommended to avoid moisture. As cable act as a long attenuator will also return loss be attenuated and will only cause minor mismatch but it can affect field strength near end of cable. No special TX protection is needed else then normal impedance matching circuit. Cable can be grounded but is usually not. Producers of leaky coaxial cables have often several different types of cables such as special designed for a certain leakage per meter, frequency range (slotted cable) and instructions how to calculate coverage. One example: http://www.solwise.co.uk/wireless-leaky-feeder-cable-rmc-50-12.htm If for example 802.11 n must at least two cables be used.
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